HR: 1340h
AN: V33C-1511    [Abstracts]
TI: Magma Recharge and Mixing Processes That Triggered the Eruption of Trachytes and Phonolites at Suswa Volcano, Kenya Rift, East Africa
AU: * Vanessa, E V
EM: vvespejel@miners.utep.edu
AF: University of Texas at El Paso, Department of Geological Sciences, 500 W. University Ave., El Paso, TX 79968, United States
AU: Anthony, E Y
EM: eanthony@utep.edu
AF: University of Texas at El Paso, Department of Geological Sciences, 500 W. University Ave., El Paso, TX 79968, United States
AU: Ren, M
EM: ren@geo.utep.edu
AF: University of Texas at El Paso, Department of Geological Sciences, 500 W. University Ave., El Paso, TX 79968, United States
AU: Macdonald, R
EM: r.macdonald@lancaster.ac.uk
AF: The Lancaster Environment Centre, Lancaster University, Lancaster, LAI 4YQ, United Kingdom
AU: White, J C
EM: john.white@eku.edu
AF: Eastern Kentucky University, Department of Geography and Geology, 521 Lancaster Avenue, Richmond, KY 40475, United States
AB: Suswa Volcano, one of a series of Holocene central vent volcanoes located in the central part of the Kenya Rift, is divided into four major stages: 1) pre-caldera; 2) syn-caldera; 3) post-caldera I; and 4) post-caldera II. In addition to central vent volcanoes are basalt and basaltic-trachyandesite (BTA) flows (e.g. Tandamara and Elmenteita) that occur in low-lying areas adjacent to the central volcanoes. Both pre- and syn-caldera rocks include trachyte to phonolite. Syn-caldera rocks also include BTA similar to Tandamara. Matrix glass in pre-caldera samples is similar to whole-rock. However, for syn-caldera samples, light and dark mixed glasses are observed petrographically and compositions range from trachyandesite to trachyte. Pre-caldera samples have a phenocryst assemblage of anorthoclase (An_0-5Ab_50-60Or_30-45), clinopyroxene (En₂₇Fs₂₈Wo₄₅) and Fe-Ti oxide. Syn-caldera trachyte-phonolite contain this same assemblage but also plagioclase (An₅₂Ab₄₅Or₃), with a composition identical to BTA samples. Clearly, the syn-caldera event represents magma mixing between BTA and trachyte. With the developing East Africa rifting, introduction of mafic magmas into the Suswa trachytic chamber was possible, similar to the rupture events in Ethiopia in 2005 (Wright et al., 2006). Post-caldera rocks have phenocrysts of alkali feldspar, olivine (Fa₇₀), clinopyroxene and Fe-Ti oxides. Post-caldera II alkali feldspars are zoned and contain two different core compositions, one with low anorthite content similar to pre- and syn-caldera samples (An₃Ab₆₄Or₃₃) and the other with higher anorthite content (An₁₇Ab₆₉Or₁₄). They exhibit oscillatory zoning, with compositional variation between Ca₂O and K₂O and have thin rims with composition similar to the matrix feldspars. The thin rims may represent magma recharge that triggered eruption of the phonolite. Matrix glass in post-caldera rocks includes both trachyandesite and phonolite, indicating that hybridization of the contrasting magmas is still ongoing. Processes in addition to mixing contribute to this dynamic volcanic setting. Post-caldera rocks have compositions of essentially all elements that are intermediate between BTA and trachyte, as appropriate to mixing. However, samples show variable Na₂0 content at constant K₂0, Fe0, MgO, and CaO. A possible explanation for this variable Na₂O content is assimilation of sodalite-bearing syenite roof and sidewall rocks into the evolving magma chamber. References: Wright, T.J., et al., 2006, Magma-maintained rift segmentation at continental rupture in the 2005 Afar dyking episode. Nature, 442: 291-294.
DE: 3618 Magma chamber processes (1036)
DE: 3619 Magma genesis and partial melting (1037)
DE: 3625 Petrography, microstructures, and textures
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting